Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and cook for 5 minutes to soften slightly. Remove and drain upside down on paper towels.
Heat olive oil in a large skillet over medium heat. Add chopped onion and cook for 3-4 minutes until translucent.
Add minced garlic and cook for 1 minute until fragrant.
Add ground beef to the skillet. Cook, breaking it apart with a spoon, until browned and fully cooked, about 6-8 minutes. Drain excess fat if necessary.
Stir in diced tomatoes, cooked rice, dried oregano, salt, and black pepper. Cook for 2 minutes to combine flavors.
Remove the skillet from heat and stir in chopped parsley and half of the shredded cheddar cheese.
Place the bell peppers upright in a baking dish. Spoon the beef and rice mixture evenly into each pepper, pressing down gently.
Pour gluten-free tomato sauce over and around the stuffed peppers in the baking dish.
Sprinkle the remaining shredded cheddar cheese on top of each stuffed pepper.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes, until the cheese is melted and bubbly.
Remove from oven and let cool for 5 minutes before serving.